There are many different welding processes. One welding process is a TIG process. Tungsten inert gas (TIG) welding is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by an inert shielding gas (argon or helium), and a filler metal is typically used. TIG welding is usually performed using CC (current controlled) power. Welding type power supply, as used herein, is a power supply that provides welding type power. Welding type power, as used herein, is power suitable for welding, induction heating, or plasma cutting.
There are many different topologies used in welding type power supplies. Switched power supplies are often used to allow for output control. One prior art welding type power supply includes a preregulator, a high voltage split bus, and a stacked inverter output, and is shown in patent application Ser. No. 13/839,235, published as US-2014-0021180-A1, hereby incorporated by reference. This power supply is well suited for controlling the output using pulse width modulation of the output inverters. Another prior art welding type power supply well suited for pwm control of the output is described in U.S. Pat. No. 8,455,794, also incorporated by reference.
Starting a TIG weld can require skill. Typical methods of starting a TIG process include using HF, lift art, and scratch start. Lift starts require specific controls, and the user touches the arc to the workpiece. As the electrode is lifted the arc is struck. HF starts are easy and reliable, but can cause interference with other equipment and requires the machine have the capability of providing an HF output. Scratch start requires no extra equipment, but requires the most skill. Scratch start involves sweeping the tungsten across the work piece.
TIG starts can “stick” and leave tungsten on the workpiece. This is particularly a problem for non-HF starts, and most particularly a problem for scratch starts. One cause of sticking is current overshoot during arc initiation. Current overshoot can happen because the power supply has a fast output current slew rate (this may be desirable at times other than the start and for other processes). Accordingly, a welding type power supply that provides reliable starts for TIG welding, while not sacrificing a fast slew rate after starting, is desired.